1. Field of the Invention
This invention relates to an optical pickup head device used for accessing data, and more particularly to a laser light source, which can produce two laser lights with different wavelengths, suitable for various uses such as compact disk (CD), CD-recordable, or digital versatile disk (DVD).
2. Description of Related Art
A DVD device was successfully developed and became a commercial product at the end of 1996. The DVD device is a highlight product of computer peripheral products because the DVD with its extremely large memory capacity is able to store image, sound, information, and multimedia, for example, in just one format. The DVD has a memory capacity of about 4.7 GB, which is much larger than the 650 MB of a typical compact disk read-only memory (CD-ROM). The DVD is predicted to be a most promising product in the upcoming years. Manufacturers are now devoting a lot of effort to further developing DVD player technology. Since the main purpose of the DVD player is to include all various media in one storage format, the DVD player should have the capability to read all currently existing types of optical disks, which includes compact disk (CD), CD-read only memory (CD-ROM), and CD-recorder (CD-R). In a CD optical pickup head, the laser light source has a wavelength of 780 nm, and the numerical aperture (NA) of the objective lens is about 0.45. However, DVD players use a laser light source with a wavelength of 635 nm or 650 nm. In order to satisfy the requirement of reading CD's as well as DVD's, several efforts were made to design an objective lens system with two different NA's, NA 0.6 for DVD and NA 0.38 for CD. The early-stage DVD player could therefore accurately focus the light to a small spot both through the 0.60 mm DVD substrate thickness and the 1.2 mm CD substrate thickness. So, early-stage DVD players could also read CD's, and thus was both DVD and CD compatible.
However, it still could not read a CD-R optical disk. Generally, the recording material on the CD-R optical disk has very low reflectivity to a laser light with a wavelength of 635 nm or 650 nm so that the CD-R can only be read by an optical pickup head having a laser light with a wavelength of 780 nm. This resulted in the inability of the early-stage DVD players to read information stored on CD-R media. Nowadays, since the CD-R optical disk is extremely popular, it has become necessary to create a DVD player that is compatible with the CD-R media. Current DVD-ROM pickup head designs usually include two lasers having wavelengths of 650 nm and 780 nm respectively, in order to also read all of the CD system products. As a result, the DVD-ROM player needs to include one more light path for a laser light with a wavelength of 780 nm. This not only increases the dimensions and complexity of the optical pickup head but also increases the fabrication cost.
FIG. 1A is a schematic drawing, illustrating a conventional structure of an optical pickup head with a single light source. In FIG. 1A, a laser light emitted from a laser diode 102 passes a diffraction grating 104 and is incident on beam splitter 106. The beam splitter 106 reflects the laser light through a collimator lens 108. The laser light is collimated and then is focused by an objective lens 110 to reach an optical disk 112 so as to read information stored on the optical disk 112. The laser light is reflected by the optical disk 112 and travels back to the beam splitter 106 through the same light path. The laser light continuously travels through a cylindrical lens 114 and finally reaches a photodetector 116.
FIG. 1B is a schematic drawing, illustrating another conventional structure of an optical pickup head with a single light source, which uses a holographic laser module to replace a number of the traditional components used in the pickup at FIG. 1A. In FIG. 1B, a holographic laser module 120 is applied. A laser light emitted from a laser diode 122 passes a holographic optical element (HOE) 124 and then enters a collimator lens 126. After the laser light is collimated, the laser light continuously travels through an objective lens 128 and is focused onto an optical disk 130 to read information stored on the optical disk 130. The laser light follows the same light path and travels back to the HOE 124. The HOE 124 deflects the laser light onto a photodetector 132.
In summary, current DVD optical pickup heads carrying a single light source with wavelength 650 nm is not compatible with the CD-R disk medium. Moreover, although the current DVD dual-wavelength optical pickup heads can read different types of optical disks like DVD, CD, CD-ROM, and CD-R, their design is too complex and fabrication cost is too high.